3. sensory system

SENSORY SYSTEM
PHYSIOLOGY: NEUROLOGY

MODALITIES OF SENSATION
OVERVIEW
‣ The sensory system includes both the central nervous system (CNS) and
the peripheral nervous system (PNS) and consists of
‣ sensory receptors at the peripheral endings of afferent neurones
‣ the ascending pathways in the spinal cord
‣ the brain centres responsible for sensory processing and perception
‣ Sensory receptors exist internally and externally around the body and
are activated via different stimuli
‣ They are designed to respond to the body’s interaction with the
external environment or its internal state

OVERVIEW
‣ Sensory receptors are specialized to respond to stimuli
including
‣ light, chemical, mechanical, thermal or nociceptive stimuli
‣ The activated sensory receptor generates an action potential,
which propagates along the axon to reach the CNS
‣ Learning Goal
‣ To consider the different types of sensory receptors and
some of their properties

SENSORY TRANSDUCTION
‣ A stimulus to a sensory receptor results in a change in cell wall’s
ionic permeability, which leads to a generation of an action
potential
‣ This process of converting a sensory signal into an electrical
signal is known as sensory transduction
‣ In general, a higher intensity stimulus will generate a higher
frequency of action potentials along the neuron
‣ However, different types of receptors will adapt to prolonged
stimulation in different ways

SENSORY TRANSDUCTION
‣ Tonic receptors are slowly adapting receptors
‣ They will respond to the stimulus as long as it persists, and produce a
continuous frequency of action potentials
‣ They convey information about the duration of the stimulus
‣ Phasic receptors are rapidly adapting receptors
‣ They will respond quickly to stimuli but stop responding upon continual
stimulation
‣ Therefore, action potential frequency decreases during prolonged stimulation
‣ This class of receptor conveys information about the changes to the stimulus
such as intensity

SENSORY MODALITY
‣ Sensory modalities can be thought of as subtypes of sensory experiences
‣ eg. pain, temperature, pressure
‣ Each sensory modality is perceived by a class of specialized receptors:
‣ Nociceptors
‣ Thermoreceptors
‣ Mechanoreceptors
‣ All modalities have different qualities that can be felt
‣ Eg. pain can be sharp/aching, the temperature can be hot/warm/cold
‣ The quality felt depends on the subtype of the activated receptor, whereas its intensity
depends on the strength of the stimulus

NOCICEPTORS
‣ Nociceptors are receptors, which respond to noxious stimuli (stimuli that would cause
tissue injury if they were to persist) and their activation results in the sensation of pain
‣ The receptors are free nerve endings, found on the ends of the type Aδ fibres and type
C fibres that transmit the pain sensation
‣ These are further categorized into:
‣ Mechanical – stimulated by the distension of skin (stretch) and pressure eg. in
inflammation
‣ Thermal – stimulated by extremities of temperature
‣ Chemical – stimulated by exogenous and endogenous chemical agents, such as
prostanoids, histamines etc
‣ Polymodal – can respond to more than one stimuli

THERMORECEPTORS
‣ Thermoreceptors are found within the skin, liver, skeletal muscle and
hypothalamus
‣ They respond to changes in temperature
‣ Those responding to warm temperatures are present within type
C fibres
‣ Those responding to cold are present within of both type C and
type Aδ fibres
‣ Cold thermoreceptors are thought to be around three times more
common than those responding to warmth

THERMORECEPTORS
‣ A warm stimulus results in both an increase in firing for warm
receptors and a decrease in firing for cold receptors (and vice
versa for cool stimuli)
‣ Initially, the response of receptors changes very quickly based on
minute temperature changes, however, after a time,
this phasic activity switches to a tonic response, allowing
adaptation to the temperature if it remains constant
‣ Temperature stimuli are transmitted to the central nervous system
via the lateral spinothalamic tract, a part of the anterolateral
system

MECHANORECEPTORS
‣ These are located in joint capsules, ligaments, tendons, muscle and skin, and respond to
deformation by the means of pressure, touch, vibration or stretch
‣ Merkel’s discs:
‣ These are tonic receptors present in skin, near the border of the dermis and
epidermis
‣ They respond to pressure and are particularly sensitive to edges, corners and points
‣ They play a key role in differentiating textures
‣ Meissner’s corpuscles:
‣ These are phasic receptors present in the dermis of the skin, namely on the palms of
the hands, soles of the feet, lips and tongue
‣ They detect the initial contact with objects or slipping of the objects held in hand

MECHANORECEPTORS
‣ Pacinian corpuscles:
‣ These are phasic receptors are present in the dermis, hypodermis,
ligaments and external genitalia
‣ They respond to high-frequency vibration and are crucial in our ability
to detect vibrations transmitted through objects in our hands
‣ Ruffini corpuscles:
‣ These are tonic receptors present in the dermis, ligaments and tendons
‣ They are the least understood of the mechanoreceptors
‣ They respond to stretch and signal position and movements of fingers

MECHANORECEPTORS
‣ Muscle spindles and Golgi tendon organs:
‣ These exist in skeletal muscle and detect stretch
‣ The detected stimuli contribute to the generation
of reflexes at the level of the spinal cord
‣ The signals are also transmitted to the dorsal column medial
lemniscal pathway (DCML)
‣ Eventually reaching the cerebral cortex and providing
information about posture, position and orientation of
limbs and joints in space –> proprioception

https://teachmephysiology.com/nervous-system/sensory-system/modalities-of-sensation/

REVIEW QUESTIONS
‣ Which are slowly adapting receptors?
‣ Tonic receptors
‣ Phasic receptors
‣ Polymodal receptors
‣ Transduction receptors

REVIEW QUESTIONS
‣ Which type of receptor responds to noxious stimuli?
‣ Nociceptors
‣ Thermoreceptors
‣ Baroreceptor

REVIEW QUESTIONS
‣ What are located in joint capsules, ligaments, tendons,
muscle and skin, and respond to deformation by the
means of pressure, touch, vibration or stretch?
‣ Nociceptors
‣ Thermoreceptors
‣ Baroreceptor

REVIEW QUESTIONS
‣ Which are phasic receptors present in the dermis of the skin (palms
of the hands, soles of the feet, lips and tongue) that detect the initial
contact with objects or slipping of the objects held in hand?
‣ Merkel’s discs
‣ Meissner’s corpuscles
‣ Pacinian corpuscles
‣ Ruffini corpuscles
‣ Muscle spindles/Golgi tendon organs

REVIEW QUESTIONS
‣ Temperature stimuli are transmitted to the central nervous
system via which tract?
‣ The lateral spinothalamic tract
‣ The dorsal column tract
‣ The anterior spinothalamic tract
‣ The medial lemniscus

PAIN PATHWAYS
OVERVIEW
‣ Pain is a somatic and emotional sensation which is unpleasant in
nature and associated with actual or potential tissue damage
‣ The classification of pain is complicated and there are many different
types of pain, each arising through unique mechanisms
‣ Types of pain include: sharp pain, prickling pain, thermal pain,
aching pain
‣ The origin of pain can be somatic, visceral, thalamic,
neuropathic, psychosomatic, referred or illusionary
‣ The nature of pain can also be acute or chronic

PAIN PATHWAYS
OVERVIEW
‣ Physiologically, the function of pain is critical for survival
‣ behaviours which cause pain are often dangerous and harmful
‣ they are generally not reinforced and are unlikely to be repeated
‣ Learning Goal
‣ To provide a general overview of a ‘classic’ picture of pain i.e. the
pain we feel when we stub our toe or touch something sharp
with a focus on how the pain pathway is initiated and processed
within the spinal cord

PAIN PATHWAYS
THE GENERAL PAIN PATHWAY
‣ Within the pain pathway there are 3 orders of neurons which carry action potentials signalling pain:
‣ First order neurons
‣ These are pseudounipolar neurons which have cells bodies within the dorsal root
ganglion
‣ They have one axon which splits into two branches, a peripheral branch (which extends
towards the peripheries) and a central branch (which extends centrally into spinal cord/
brainstem)
‣ Second order neurons
‣ The cell bodies of these neurons are found in the Rexed laminae of the spinal cord, or in
the nuclei of the cranial nerves within the brain stem
‣ These neurons then decussate in the anterior white commissure and ascend cranially in
the spinothalamic tract to the ventral posterolateral (VPL) nucleus of the thalamus

PAIN PATHWAYS
THE GENERAL PAIN PATHWAY
‣ Third order neurons
‣ The cell bodies of third order neurons lie within the VPL of the
thalamus
‣ They project via the posterior limb of the internal capsule to
terminate in the ipsilateral postcentral gyrus (primary
somatosensory cortex)
‣ The postcentral gyrus is somatotopically organized
‣ Therefore, pain signals initiated in the hand will terminate in the
area of the cortex dedicated to represent sensations of the hand

https://teachmephysiology.com/nervous-system/sensory-system/pain-pathways/

PAIN PATHWAYS
NOCICEPTORS
‣ Some first order neurons have specialist receptors called
nociceptors which are activated through various noxious stimuli
‣ Nociceptors exists at the free nerve endings of the primary
afferent neuron
‣ Since nociceptors are free nerve endings this means they
are unencapsulated cutaneous receptors
‣ This is opposed to encapsulated cutaneous receptors (e.g.
Merkel’s discs) which detect other sensory modalities such as
vibration and stretching of skin

PAIN PATHWAYS
NOCICEPTORS
‣ Similar to other sensory modalities, each nociceptor has its own receptive field
‣ This means one nociceptor will transduce the signal of pain when a particular
region is skin is stimulated
‣ The size of receptive fields vary throughout the body and there is often overlap
with neighbouring fields
‣ Areas such as the fingertips have smaller receptive fields than areas such as the
forearm
‣ In addition, they have a larger density of free nerve endings within this receptive
field
‣ This difference is important as it allows for greater acuity in detecting a sensory
stimulus

PAIN PATHWAYS
NOCICEPTORS
‣ The size of cortical representation in the somatosensory
cortex of a particular body part is also related to the size of
the receptive fields in that body part
‣ For example, because the fingertips have small receptive
fields, and thus a greater degree of sensory acuity, they
have a larger cortical representation
‣ Nociceptors can be found in the skin, muscle, joints, bone
and organs (other than the brain) and can fire in response
to a number of different stimuli

PAIN PATHWAYS
NOCICEPTORS
‣ Three types of nociceptors exist:
‣ Mechanical nociceptors
‣ detects sharp, pricking pain
‣ Thermal and mechano-thermal nociceptors
‣ detects sensations which elicit pain which is slow and
burning, or cold and sharp in nature
‣ Polymodal nociceptors
‣ detects mechanical, thermal and chemical stimuli

PAIN PATHWAYS
TRANSMISSION TO THE SPINAL CORD
‣ Signals from mechanical, thermal and mechano-thermal nociceptors
are transmitted to the dorsal horn of the spinal cord predominantly
by Aδ fibres
‣ These myelinated fibres have a low threshold for firing and the fast
conduction speed means they are responsible for transmitting the first
pain felt
‣ In addition, Aδ fibres permit for the localization of pain and form the
afferent pathway for the reflexes elicited by pain
‣ Aδ fibres predominantly terminate in Rexed laminae I where they
mainly release the neurotransmitter glutamate

PAIN PATHWAYS
‣ Polymodal nociceptors transmit their signals into the
dorsal horn through C fibres
‣ C fibres are unmyelinated and a slow conduction speed
‣ For this reason, C fibres are responsible for the secondary
pain we feel which is often dull, deep and throbbing in
nature
‣ These fibres typically have large receptive fields and
therefore lead to poor localization of pain

PAIN PATHWAYS
‣ Compared to Aδ fibres, C fibres have a high threshold for firing
‣ However, noxious stimuli can cause sensitization of C fibres
and reduce their threshold for firing
‣ C fibres predominately terminate in Rexed laminae I and II and
release the neurotransmitter substance P
‣ Other neurotransmitters are released by primary afferent
neurons terminating within the spinal cord such as aspartate
and vasoactive peptide

PAIN PATHWAYS
‣ A variety of factors are released upon tissue damage which lead to the activation of nociceptors
‣ These include:
‣ Arachidonic acid
‣ Potassium
‣ 5-HT
‣ Histamine
‣ Bradykinin
‣ Lactic acid
‣ ATP
‣ Many of these factors are also proinflammatory and lead to acute inflammation in the area of
damage

PAIN PATHWAYS
ALTERED PERCEPTION OF PAIN: HYPERALGESIA
‣ Hyperalgesia describes the phenomenon where there is an enhanced
sensation of pain at normal threshold stimulation
‣ The pathophysiology of hyperalgesia is believed to arise from the
sensitization of nerves in and around the damaged area due to the
release of molecules such as prostaglandin E2
‣ In addition, free nerve endings release substance P which acts on
surrounding cells causing them to release molecules which potentiate
pain
‣ Sensitization means that these nerves have a reduced threshold for
firing as a result of noxious stimuli

PAIN PATHWAYS
ALTERED PERCEPTION OF PAIN: HYPERALGESIA
‣ Sensitization can also occur centrally within the spinal cord
‣ The pathophysiology is thought to arise from an increase in
number of NMDA receptors, as well as increased sensitivity of
NMDA receptors to glutamate
‣ These changes occur on the dendrites of second order neurons
and are a result of prolonged glutamate release from first order
neurons due to persistent nociceptor activation
‣ Central sensitization leads to hyperexcitability of second order
neurons within the dorsal horn of the spinal cord

PAIN PATHWAYS
ALTERED PERCEPTION OF PAIN: ALLODYNIA
‣ Allodynia is where pain is felt on a stimulus which was
previously not painful
‣ Allodynia is also observed in and around areas effected by
noxious stimuli
‣ Hyperalgesia and allodynia are features of a physiological
response to pain but can also be observed in pathology
‣ For example, neuropathic pain is characterized by hyperalgesia,
allodynia and spontaneous firing of nociceptors due to nerve
damage

PAIN PATHWAYS
DESCENDING MODULATION OF PAIN
‣ Within the central nervous system there are three types of opioid receptors which regulate
neurotransmission of pain signals
‣ These receptors are called mu, delta and kappa opioid receptors
‣ They are all G protein coupled receptors and their activation leads to a reduction in
neurotransmitter release and cell hyperpolarization, reducing cell excitability
‣ Exogenous opioids, such as morphine, provide excellent analgesia by acting on these receptors
‣ Likewise, our body contains endogenous opioids which can modulate pain physiologically
‣ There are three types of endogenous opioids:
‣ Β-endorphins – which predominately binds to mu opioid receptors
‣ Dynorphins – which predominately bind to kappa opioid receptors
‣ Enkephalins – which predominately bind to delta opioid receptors

PAIN PATHWAYS
DESCENDING MODULATION OF PAIN
‣ Opioids can regulate pain on a number of levels, both within the spinal cord, brain
stem and cortex
‣ Within the spinal cord both dynorphins and enkephalins can act to reduce the
transmission of pain signals in the dorsal horn
‣ Opioid receptors are located in
‣ The pre-synaptic membrane of second order neurons
‣ The post-synaptic membrane of first order neurons
‣ When endogenous opioids act on these receptors it reduces neurotransmitter release
from the first order neuron, and causes hyperpolarization of the second order neuron
‣ Together, this reduces the firing of action potentials in the second order
neuron, blocking the transmission of pain signals

PAIN PATHWAYS
REVIEW QUESTIONS
‣ How many orders of neurons are there in the pain
pathway?
‣ Where do they begin and end?

PAIN PATHWAYS
REVIEW QUESTIONS
‣ Which type of nociceptor detects sensations which elicit
pain that is sharp or pricking in nature?
‣ Mechanical nociceptors
‣ Thermal nociceptors
‣ Mechano-thermal nociceptors
‣ Polymodal nociceptors

PAIN PATHWAYS
REVIEW QUESTIONS
‣ In the pathology of hyperalgesia, free nerve endings
release what compound which acts on surrounding cells
causing them to release molecules which potentiate pain?
‣ Sodium
‣ Dopamine
‣ Substance P
‣ Endorphins

SENSORY ACUITY
OVERVIEW
‣ Sensory acuity signifies how accurately a stimulus can be located
‣ Its degree varies between areas of the body depending on function
‣ eg. the fingertips require a greater sensory acuity than the
forearm
‣ This is determined by 3 things:
‣ 1. lateral inhibition of the central nervous system (CNS)
‣ 2. two-point discrimination
‣ 3. synaptic convergence and divergence

SENSORY ACUITY
LATERAL INHIBITION
‣ Lateral inhibition is the ability of excited neurones to inhibit the activity of
neighbouring neurones
‣ This prevents the spread of neuronal activity laterally
‣ Consequently, there exists an increased contrast in excitation between
neighbouring neurones, allowing better sensory acuity
‣ Lateral inhibition is a key component of sensory processing
‣ It is most important in visual processing as it helps to increase the contrast
and enhance the perception of edges
‣ Lateral inhibition is also important when processing fine touch, as the amplified
differences in neuronal activity allow a person to better pinpoint the area being
touched

https://teachmephysiology.com/nervous-system/sensory-system/sensory-acuity/

SENSORY ACUITY
TWO POINT DISCRIMINATION
‣ Two-point discrimination is the ability to discern between two points touching the skin
‣ It describes the minimum distance required between two simultaneous stimulations applied in
order to be registered
‣ This distance tends to vary depending on which part of the body we are testing
‣ In the fingertips, the minimum distance required to achieve this phenomenon is smaller than
the distance required further up the arm
‣ Two factors determine two-point discrimination:
‣ density of sensory receptors
‣ size of neuronal receptive fields
‣ The higher the number of sensory receptors in a region, the more accurate the sensory perception
of the region
‣ Fingertips have 3-4 times more density of sensory receptors than the hand

SENSORY ACUITY
TWO POINT DISCRIMINATION
‣ Each neurone has a specific sensory space that if simulated will result in the
activation of that particular neurone
‣ This space is known as the receptive field of that neurone
‣ The receptive field varies in size
‣ Larger -> greater area in which it detects changes
‣ Larger -> less precise perception
‣ Areas with the most sensitive two-point discrimination should have a high
density of receptors with small receptive fields
‣ Receptive fields can overlap which also results in reduced precision of
perception

SENSORY ACUITY
SYNAPTIC CONVERGENCE AND DIVERGENCE
‣ Synaptic convergence is when several first order neurones converge on
one second order neurone
‣ It results in reduced acuity as signals from multiple receptors converge
on one neurone
‣ This process also increases the overall sensitivity to stimulation as
activation of multiple receptors can activate the second order neurone
‣ Synaptic divergence involves one first order neurone stimulating several
second order neurones
‣ It allows for a more precise sense of perception by amplifying the
signal from a single receptor or area

SENSORY ACUITY
REVIEW QUESTIONS
‣ What would need to be done to enhance our ability to discern between
two different objects touching our back?
‣ Increase the receptive fields of individual receptors and increase the
receptor density.
‣ Increase the receptor density only. The size of the receptive field does
not influence this phenomenon.
‣ Increase the receptor density and reduce the receptive field of
individual receptors.
‣ Reduce both the receptive field of individual receptors and the
receptor density.

SENSORY ACUITY
REVIEW QUESTIONS
‣ The process of edge enhancement while perceiving visual
stimuli is mostly achieved by:
‣ Synpatic convergence
‣ Lateral inhibition
‣ Synaptic divergence
‣ Two-point discrimination

SENSORY ACUITY
REVIEW QUESTIONS
‣ Which of the following body parts has the most accurate
two-point discrimination?
‣ Back
‣ Arm
‣ Fingers
‣ Tongue

References
These slide reflect a summary of the contents of
TeachMePhysiology.com and are to be used for
educational purposes only in compliance with the terms of
use policy.
Specific portions referenced in this summary are as follows:
‣ https://teachmephysiology.com/nervous-system/sensory-system/modalities-of-
sensation/
‣ https://teachmephysiology.com/nervous-system/sensory-system/pain-pathways/
‣ https://teachmephysiology.com/nervous-system/sensory-system/sensory-acuity/
Additional sources are referenced on the slide containing
that specific content.

3. sensory system

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